1. Field of the Invention
The present invention relates to a semiconductor memory device. More particularly, the invention relates to a magnetic memory device having memory cells that include magnetoresistive effect elements.
2. Description of the Related Art
Magnetic memory devices are known as one kind of a semiconductor memory device. Various types of magnetic memory devices have been proposed. In recent years, magnetic random access memories (MRAM) have been proposed, which comprise magnetoresistive effect elements that exhibit a giant magnetoresistive effect. Particularly, MRAMs that have ferromagnetic tunnel junction attract attention. Since MRAMs are highly nonvolatile, operate at high speed, have high integration density and are highly reliable, they are regarded as memory devices that may replace dynamic random access memories (DRAM), electrically erasable programmable read only memories (EEPROM) and are being developed. See Mark Durlam et al., “A 1-Mbit MRAM Based on 1T1MTJ Bit Cell Integrated With Copper Interconnects,” IEEE Journal of Solid-State Circuits, Vol. 38,No. 5, May 2003, pp. 759-773, and J. DeBrosse et al., “A 16 Mb MRAM Featuring Boostrapped Write Devices,” 2004 Symposium on VLSI Circuits Digest of Technical Papers, pp. 454-457.
Magnetic tunnel junction (MTJ) elements for use in the memory cells of the MRAM have, for example, a first ferromagnetic layer, a second ferromagnetic layer, and an insulating layer interposed between the ferromagnetic layers. An MTJ element has a tunneling magnetoresistive effect; its tunneling current changes in accordance with whether the spin directions of the ferromagnetic layers are parallel or anti-parallel. In the MRAM, each memory cell stores data 0 when the MTJ element has low resistance, and data 1 when the MTJ element has high resistance.
In most MRAMs, a current is made to flow in each write line, generating a magnetic field. Data is thereby written into an MTJ element connected to the write line. That is, the data is indirectly written into the MTJ element, not directly as into the memory cells of ordinary semiconductor memories. Inevitably, a relatively large current must flow in the write line to write the data into the MTJ element. To be more specific, a current of about several milliamperes to tens of milliamperes flow in the write line.
To supply this large write current to any selected write line, the MRAM must have a large current-supplying circuit, a large current-sinking circuit, and a large switch circuit for supplying the current to, and sinking it from, the selected write line. Consequently, the MRAM will be a large chip, and the manufacturing cost of the chip will increase. Further, the circuit (i.e., transistors) connected to the write lines must be large, unavoidably increasing the parasitic capacitance of the write lines, inevitably increasing the data-reading speed if the write lines are used as read lines, too. To increase the data-reading speed, read lines may be provided in addition to the write lines, as is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2002-170376.